A mobile telephone exchange is linked up to the telephone line network. This exchange serves a plurality of radio transmitters serving as base stations B1, B2, B3 having a limited range. The area a base station serves is called a cell, thus the name "cellular system" (FIG. 1). The different cells are denoted C1, C2, C3, . . .
The cells are divided into clusters for achieving a given interference distance between cells with the same frequency. The size of the clusters is different depending on the signal-to-noise ratio which is sought, but a usual number of cells in a cellular cluster is 21 (in theoretical calculations also 3, 7, 9 and 12). Each cluster uses all frequencies, distributed over its stations. Apart from the speech channels there is also one or more call channels per cell, which is used to localize the mobile stations (mobiles) and send messages between base and mobile.
When a speech is to be connected to or from a mobile M, the exchange pages, i.e. it searches for the position of the mobile with the aid of the call channel. Connection to the nearest base and selection of channel subsequently takes place. The speech is blocked if there is no unoccupied channel. If the mobile is moving during the established call, it sometimes occurs that it passes a cell boundary G. The exchange automatically performs a handover, i.e. transfer of the established call from the base B1 to the new base B4.
When fixed frequency allocation is used, each base has access to a plurality of channels for establishing calls. If all channels are occupied when a new call arrives, the latter must be blocked. To avoid interference, cells with the same frequency must have a given interference distance to each other.
In adaptive channel selection, fixed frequency allocation is not used for the bases but the frequency allocation can vary in response to measured signal-noise ratios, see below.
In the adaptive channel selection, no consideration is paid to how great the interference distance is, and it is important that the calls do not interfer with each other. Theoretically, if not practically, the mobiles can be placed side by side and send on the same frequency.
The basis for the adaption are the signal-noise ratios (C/I) measured in the bases and mobiles. The signal paths between a mobile and its base MS-BS and an interfering mobile-base-pair MSs-BSs are shown in FIG. 2. S.sub.1 -S.sub.4 is the received signal strength in the respective communication. They are equal in both directions, apart from Rayleigh fading.
Connection establishment always takes place to the base which is best according to some criterion, e.g. the best base being the one with the strongest signal. If the base does not have any unoccupied channels, MS can be ordered to make a new selection (next best).
After measuring all signal-noise-ratios, selection of a channel having the greatest C/I value is carried out. For each channel, the lowest of the four C/I values is used, but a simpler alternative can also be envisaged where only the C/I values for BS and MS are considered. No consideration is then taken to whether there will be interference in existing connections. This alternative is used in the embodiment below.
Handover takes place on two occasions:
1. When the quality falls below the minimum limit which has been set, the call must be transferred to a better channel. This is carried out as an ordinary channel selection, but if no sufficiently good channel is available, the call changes base station and makes a new channel selection.
2. When the used base is no longer best or when no acceptable channel is available, change of base takes place. A request for handover can be sent from MS via the old BS to the new one via the telephone network, or directly between MS and the new BS via the call channel.